The motion pattern of a healthy joint is complex and involves rotation and translations, sometimes alone and sometimes in combination. Accordingly, the instantaneous center or axis of rotation moves along with the joint. Typical conventional arthoplasty implants are typically limited in their ability to realize complex motions that would better replicate the physiological motion of the intact anatomical structures, and thus do not respond accurately to anatomical loads.
For example, conventional ball-on-socket articulations are limited to rotations, provide only a limited amount of lateral translation and no axial damping. Moreover, such implants tend to generate wear particles that can cause osteolysis in the neighbouring bony structures or induce immune response in case of toxic materials. So-called artificial discs based on a soft core between two rigid plates provide more motion, however the long term stability of the involved polymers is still not well understood. Still other implants have been found to rotate, translate, or undergo other undesirable motion, for instance when placed under compression.